Atomic and Molecular Scattering from Solids. II. Comparison of Classical Scattering Models in Relation to Experiment

Abstract

Characteristics of a classical two–dimensional (2d) model of atomic scattering from finite temperature, elastic solids are compared to similar characteristics of “quasi-1d” scattering models in light of pertinent experimental scattering studies of rare-gas atomic beams on metals. (The term “quasi-1d” is used to denote scattering models based on the assumption that tangential interaction is negligible, i.e., dynamical interaction is presumed to occur only in the direction normal to the local surface plane.) The 2d finite-range interaction field (denoted 2d FRI) is composed of additive Lennard-Jones (12, 6) pairwise potentials. Compared scattering characteristics include beam directivity and position as well as variation of mean tangential/normal momentum with scattering angle. It is shown that combined “structure” and “thermal” broadening associated with 2d FRI can account for systematic differences between experimentally observed lobe directivity and that predicted by quasi-1d “soft/hard cube” scattering models. For nongrazing incidence, strong angular variation of tangential momentum over the high-intensity portion of the scattered beams, as predicted by the 2d FRI model, is also in qualitative agreement with experiment. Dominant effects of finite “collision time,” as determined by 2d FRI, cast doubt on previous interpretation that observed differences in lobe position for effusive incident beams of different rare gases scattered from a particular hot metal crystal result from simple mass ratio effects. Effects of finite “collision time” are also invoked to interpret recent experimental findings on differences in lobe position for a given rare-gas beam scattered from different hot metal crystals. It is concluded that quantitative theoretical studies of gas–surface scattering must await development of tractable 3d FRI models.